It is necessary to test various industrial substances, such as, food, pharmaceuticals, cosmetics and water for microbial contamination. One area of biological testing of food, dairy, pharmaceutical, cosmetic and related types of products involves the estimation of total numbers of bacteria, yeasts and molds as well as concentrations of specific groups of organisms within the material. One widely used method is known as the "Standard Plate Count" method and involves culturing a diluted sample of the product in an agar growth medium. The plates containing the sample and the growth medium are incubated (e.g., 32.degree. C.-40.degree. C.) for 24 hours to 5 days depending upon the assay. After incubation, colonies of microorganisms which have grown in the agar are counted.
Colorimetric methods have been successfully used to classify microorganisms in clinical samples (e.g., PASCO by Difco, Detroit, Mich.). Although it would be desirable to utilize a colorimetric method, or any other optical method, for detecting microbial growth in industrial samples, the solid substances of the test samples disposed in an aqueous media usually cause optical interference for a detection system. More specifically, when solid substances are disposed in a media to allow for culturing microorganisms, the colorimetric detection system must pass light either through or reflect light from the media containing the solid substance. In most of the cases the solid substances interfere with the spectral characteristics of the media, yielding a poor signal to noise ratio of the detection system.
The present invention provides a solution to the above-discussed problems by providing a detection system which excludes interferences from any substance in the zone of the device through which the measurement is made.
A device for continuously monitoring the biological activity in a specimen is described in U.S. Pat. No. 4,945,060, issued Jul. 31, 1990. The present invention employs a different approach and thereby provides a substantial improvement over this known device in the simplicity of its manufacture and ease of using the device without compromising the integrity of the test results.